Partitioning of penoxsulam, a new sulfonamide herbicide.

Penoxsulam (trade name Granite) is a new acetolactate synthase (ALS) inhibitor herbicide for postemergence control of annual grasses, sedges, and broadleaf weeds in rice culture. This study was done to understand the equilibrium phase partitioning of penoxsulam to soil and air under conditions simulating California rice field conditions. Partitioning of penoxsulam was determined between soil and water (Kd) by the batch equilibrium method and between air and water (K(H)) by the gas-purge method. In four representative soils from the Sacramento Valley, the Kd values ranged from 0.14 to 5.05 and displayed a modest increase with soil pH. In soil amended with manure compost, soil sorption increased 4-fold with increasing soil organic matter content, but was still low with a Kd of 0.4 in samples with high organic carbon contents of 15%. Penoxsulam was confirmed to be extremely nonvolatile and did not partition into air at any measurable rate at 20 or 40 degrees C. K(H) (pH 7) was estimated at 4.6 x 10(-15) Pa x L x mol(-1) on the basis of available water solubility and vapor pressure data. The results imply that soil and air partitioning of penoxsulam do not significantly affect its potential for degradation or offsite movement in water.